The invention relates to an electric lamp having a gastight inner bulb for converting electrical energy into light, the lamp also having an outer bulb surrounding and spaced apart from the inner bulb, the lamp also having a lamp cap connected to the outer bulb, and two current conductors extending from the inner bulb, carried by a support, and electrically connected to the lamp cap, the lamp also having means designed for switching off the lamp in the case of a defect of the outer bulb.
Such a lamp is known from U.S. Pat. No. 4,229,678 (Petro), the lamp being a high-pressure mercury vapor discharge lamp. Such lamps may form a safety risk if the outer bulb of the lamp breaks while the lamp continues burning. In such a case, in fact, there is the particular risk that the high temperature and high pressure inside the inner bulb will cause the latter to break as well, so that hot substances are released into the atmosphere, with all the possible disadvantages this involves for any persons and objects that happen to be in the vicinity. Petro proposes a solution for this problem by providing a switch for switching off the lamp if the outer bulb should inadvertently be fractured. This switch is present in the top inside the outer bulb, connected in series in the electrical circuit of the lamp. The switch comprises an electrically conducting element which under normal circumstances bears under spring pressure on the inner surface of the outer bulb and which springs outward upon a fracture of the outer bulb, thus breaking the electrical circuit in the lamp. Switching-off of the known lamp in the case of an outer bulb fracture accordingly takes place through interruption of the electrical circuit inside the lamp.
A disadvantage of this solution is that the switch used therein gives the lamp an unattractive appearance, while furthermore there is an undesirable optical disturbance because the switch forms an optical barrier for the light issuing from the inner bulb to the exterior. A further disadvantage of the known lamp is that mounting of the switch is cumbersome and expensive in mass manufacture, which renders this lamp uninteresting from an economic point of view.
It is an object of the invention to counteract the disadvantages of the prior art indicated above, by providing means for extinguishing an electric lamp upon the ocurrence of a defect (for example, a fracture) of the outer bulb, in a simple, elegant, and accordingly economically responsible manner, without any undesirable optical side effects.
According to the invention, an electric lamp of the kind mentioned in the opening paragraph is characterized in that said means are constructed for short-circuiting the current conductors upon the ocurrence of a defect of the outer bulb. The short-circuit preferably activates one or several fuses present in the lamp cap, so that switching-off of the lamp can take place in a reliable but safe manner, for example, through an interruption or melting of the fuse(s).
In a preferred embodiment of a lamp according to the invention, said means are at least substantially formed by a switch which is brought from an idle state into a short-circuit state upon fracture of the outer bulb. In particular, said switch is at least substantially formed by a resilient element which in the idle state rests against the inner surface of the outer bulb and which in the short-circuit state short-circuits the two current conductors. Preferably, the resilient element is already electrically connected to one of the current conductors when in the idle state.
In a further preferred embodiment of a lamp according to the invention, said resilient element is held under spring pressure in the idle state by means of an oxygen-binding, evaporating getter which releases the spring upon fracture of the outer bulb. This has the additional advantage that the lamp is also extinguished by the resilient element if the outer bulb does not break, but does show a hole, which means that the lamp nevertheless also forms a safety risk. The outer bulb will be gastight if such a getter is used.
In a further preferred embodiment of the lamp according to the invention, said switch is at least substantially formed by a resilient element which is brought from an idle state into a short-circuit state upon an impact load on the lamp. The switch preferably comprises a rod element which is electrically connected to one of the current conductors and a ring element which is electrically connected to the other current conductor, said rod element in the idle state being inside the ring of the ring element and making electrical contact with the ring in the short-circuit state through a lateral movement, thus short-circuiting the two current conductors. This arrangement results in a short circuit of the current conductors in the event of a force exerted on the lamp, irrespective of whether the outer bulb actually breaks, has a hole, or has a crack. In other words, any unacceptable force exerted on the lamp which involves a safety risk is detected and leads to switching-off of the lamp.
It is noted in this connection that the present invention has no limitation regarding the type of electric lamp, i.e. it may be implemented in a so-called tungsten halogen incandescent lamp as well as in a gas discharge lamp. The former type is, for example, a halogen incandescent lamp with bromine, chlorine, hydrogen, and also a rare gas and/or nitrogen as the filling gas, while the latter type may be, for example, a high-pressure mercury vapor discharge lamp. Although the outer bulb need not necessarily be gastight in the case of a halogen incandescent lamp, the safety risk described above is present in both types of lamps when the outer bulb is fractured. As was noted before, the present invention is accordingly applicable to both types.
The invention also relates to a method of manufacturing an electric lamp according to the invention, which method is characterized in that said means are constructed for short-circuiting the current conductors upon a defect of the outer bulb.